Crop responses to applied soil compaction and to compaction repair treatments

Crop responses to annual compaction treatments (applied to whole plots) and management treatments to ameliorate compacted soil were deter-mined in a f ield experiment on a Vertisol. Initially, all treatments except a control w ere compacted with a 10 Mg axle load on wet soil (26% gravimetric water con tent compared with a plastic limit of 22%). Annually applied axle loads of 10 and 6 Mg on wet soil (25-32% soil water) tended to reduce seedling emerg ence, grain yield (wheat, sorghum and maize). soil water storage and crop w ater use efficiency (WUE). Annual applications of an axle load of 6 Mg on d ry soil (<22% soil water) had little effect on crop performance. Mean reduc tions in the yield of five crops (three wheat, one sorghum and one maize) i n comparison with the uncompacted control were 23% or 0.79 Mg ha(-1) (10 Mg on wet soil), 13% or 0.44 Mg ha(-1) (6 Mg on wet soil) and 1% or 0.03 Mg h a(-1) (6 Mg on dry soil). Maize grown in the fifth year of treatment applic ation was most affected by compaction of wet soil, its WUE being reduced fr om 14.3 to 9.7 kg ha(-1) mm(-1) in response to an axle load of 10 Mg. Reduc ed WUE was associated with delayed soil water extraction at depth. A 3-year pasture ley was the most successful amelioration treatment. A wheat and a maize crop grown after the ley outyielded the control by 0.33 and 0.90 Mg h a(-1), respectively. So the pasture not only ameliorated the initial compac tion damage, with respect to crop performance, but resulted in improvements in two subsequent crops. (C) 2001 Elsevier Science B.V. All rights reserve d.